This invention pertains to microlithography in which a pattern, defined on a mask or reticle, is transferred to a suitable substrate using a charged particle beam such as an electron beam. This type of microlithography has especial utility in the fabrication of semiconductor integrated circuits and displays. More particularly, the invention pertains to achieving accurate pattern-feature linewidths in the microlithographically projected pattern image even if beam-edge resolution is relatively poor.
In recent years, as semiconductor integrated circuits increasingly have become miniaturized, the resolution limits of optical microlithography (i.e., microlithography performed using ultraviolet light as an energy beam) increasingly have become apparent. As a result, considerable development effort currently is being expended to develop microlithography apparatus and methods that employ an alternative type of energy beam that offers prospects of better resolution than optical microlithography. One candidate microlithography technology utilizes a charged particle beam, such as an electron beam or ion beam, as an energy beam. The charged particle beam passes through a charged-particle-beam (CPB)-optical system from a source (e.g., electron gun) through a reticle to a substrate (e.g., semiconductor wafer).
In conventional electron-beam microlithography, the beam-edge resolution of the electron-optical system desirably is no more than one-half to one-third the minimum linewidth of the pattern as imaged on the substrate. xe2x80x9cBeam-edge resolutionxe2x80x9d is defined as the lateral distance over which the intensity of the beam at the beam edge increases from 12% to 88%. The lower the number denoting beam-edge resolution, the more sharply defined the beam edges. Hence, for example, if the minimum linewidth of the elements (features) of a pattern is 100 nm, then the beam-edge resolution desirably is 50 nm to 33 nm or less. However, a large beam current (i.e., a beam current of approximately 20 xcexcA or more) can cause the beam-edge resolution to be greater than the required value due to the influence of space-charge effects. As used herein, xe2x80x9cbeam currentxe2x80x9d refers to the total current of the electron beam reaching a sensitive substrate at any one instant. A xe2x80x9cspace-charge effectxe2x80x9d is a phenomenon in which similarly charged particles (e.g., electrons) in the beam repel each other in response to Coulomb forces between the similarly charged particles, resulting in beam spreading and consequent blurring (loss of beam-edge resolution) of the edges of the beam.
According to conventional practice, space-charge effects can be reduced by reducing the area of the substrate illuminated by the beam at any one instant and/or by reducing the beam current. Unfortunately, these tactics reduce xe2x80x9cthroughputxe2x80x9d (number of semiconductor wafers that can be microlithographically processed per unit time) to impractical levels.
In view of the shortcomings of the prior art as summarized above, an object of the invention is to provide microlithography (pattern-transfer) methods that achieve accurate pattern-feature linewidths even if beam-edge resolution is relatively poor.
To such end, and according to a first aspect of the invention, methods are provided for performing microlithography of a pattern, defined on a reticle and having a minimum linewidth, to a sensitive substrate using a charged particle beam. According to a representative embodiment of such a method, a region of the reticle is illuminated with a charged-particle illumination beam passing through an illumination-optical system. The illumination beam passing through the illuminated region of the reticle forms a patterned beam propagating downstream of the reticle. The patterned beam is projected and focused, with demagnification, through a projection-optical system onto a corresponding region on a sensitive substrate. A minimum linewidth of the pattern defined by the reticle is determined. The projection-optical system is controlled to provide the patterned beam with a beam-edge resolution that is 0.8 to 1.0 times the minimum linewidth of the pattern.
If the beam-edge resolution is as noted above, linewidth accuracy and precision can be maintained at a value that is sufficiently better than the target value of xc2x110%, especially so long as the variation in the threshold value is maintained at xc2x11% or better.
According to another representative embodiment of methods according to the invention, the pattern as defined on the reticle is divided into multiple subfields. The subfields are illuminated successively with a charged-particle illumination beam to form a patterned beam propagating downstream of the reticle. The patterned beam from each subfield is projected and focused, with demagnification, by passage through a projection-optical system so as to form images of the subfields on respective regions on a sensitive substrate such that the images are stitched together. A predicted beam-edge resolution of the projection-optical system is calculated, based on a beam current used to illuminate each subfield. Before exposing a subfield on the reticle, dimensions of pattern features as defined in the subfield are corrected according to a ratio of pattern-feature dimension to the beam-edge resolution, so as to project the pattern features in the subfield with correct pattern-feature dimensions on the substrate.
By correcting the dimensions of the pattern features on the reticle beforehand, accuracy and precision of pattern dimensions can be increased, even in instances in which beam-edge resolution is relatively coarse.
According to yet another representative embodiment of a method according to the invention, the pattern, as defined on the reticle, is divided into multiple subfields. The subfields are illuminated successively with a charged-particle illumination beam to form a patterned beam propagating downstream of the reticle. The patterned beam is projected and focused, with demagnification, from each subfield through a projection-optical system so as to form images of the subfields on respective regions on a sensitive substrate such that the images are stitched together. A predicted beam-edge resolution of the projection-optical system is calculated, based on a beam current used to illuminate each subfield. The calculated beam-edge resolution is calculated as a function of a position, within a field of the projection-optical system, in which an image of the subfield is to be formed on the substrate. Before exposing a subfield on the reticle, dimensions of pattern features as defined in the subfield are corrected according to a ratio of pattern-feature dimension to the beam-edge resolution, so as to project the pattern features in the subfield with correct pattern-feature dimensions on the substrate.
Another representative embodiment is directed to a method for manufacturing an electronic device that includes at least one layer having a pattern formed by charged-particle-beam microlithography. (The microlithography involves illumination of a reticle, defining the pattern, by an illumination beam to form a patterned beam, and transferring of the pattern from the reticle to a sensitive substrate by reducing, projecting, and focusing the patterned beam onto the sensitive substrate. The method includes a method for controlling beam blur. In the latter method, the patterned beam is passed through a projection-optical system as the patterned beam propagates from the reticle to the sensitive substrate. A beam-edge resolution that is achievable with the projection-optical system is determined, and the projection-optical system is controlled such that a poorest beam-edge resolution achieved by the projection-optical system is 0.8 to 1.0 times a minimum linewidth of the pattern.
In another representative embodiment of a method for manufacturing an electronic device that includes at least one layer having a pattern formed by charged-particle-beam microlithography, a reticle defining the pattern is segmented into multiple subfields each defining a respective portion of the pattern. The subfield images are individually and sequentially projected onto the sensitive substrate in a manner in which images of the subfields on the sensitive substrate are stitched together. The method includes a method for forming an image on the sensitive substrate of the subfield in which the pattern-feature dimension is correctly formed. According to the latter method, the patterned beam is passed through a projection-optical system as the patterned beam propagates from the reticle to the sensitive substrate. A predicted value of beam-edge resolution of the projection-optical system is determined, based on a beam current used to expose each subfield on the reticle. Before exposing a subfield, a dimension of a pattern feature defined in the subfield is corrected according to a ratio of the pattern-feature dimension to the beam-edge resolution.
In yet another representative embodiment of a method, according to the invention, for manufacturing an electronic device that includes at least one layer having a pattern formed by charged-particle-beam microlithography, the reticle is segmented into multiple subfields, each defining a respective portion of the pattern. The subfields are individually and sequentially projected onto the sensitive substrate in a manner in which images of the subfields on the sensitive substrate are stitched together. This method also includes a method for forming an image of a subfield in which pattern-feature dimensions are correct. The latter method comprises passing the patterned beam through a projection-optical system as the patterned beam propagates from the reticle to the sensitive substrate. A predicted value of beam-edge resolution of the projection-optical system is determined, wherein the value is a function of a position, within a field of the projection-optical system, where the subfield pattern is transferred. Based on the determination and before projecting an image of a subfield, dimensions of the pattern features in the subfield are corrected according to a ratio of pattern-feature dimension to beam-edge resolution.
The foregoing and additional features and advantages of the invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings.